1. Field of the Invention
This invention relates to a filter, and in particular a combline filter.
2. Description of the Prior Art
Within the communications industry, and in particular base station design, a filter that has become increasingly popular is the combline filter. The combline filter comprises a series of filter elements where each filter element has a resonator post. The coupling between different resonator posts is achieved by way of fringing fields using air as a dielectric, as described in xe2x80x98Combline band-pass filters of narrow or moderate bandwidthxe2x80x99, The Microwave Journal, Vol 6, pg 82-91, Aug. 1963. Some of the characteristics of the combline filter that have resulted in the increased popularity of the filter are low insertion losses, high Q, good out of band performance and the filters are relatively cheap to manufacture.
These filters, however, are relatively large making them unsuitable for the miniaturization of base stations for office use. Further, the required distance between two resonator posts can inhibit the required electrical coupling between adjacent resonator posts. This has resulted in the use of extended probes to provide the electrical coupling.
Ceramic filters having the required pass bands for mobile communication offer a reduction in filter size compared with a combline filter but suffer from poor out of band performance. Further, with ceramic filters it can be difficult to obtain the required electrical and magnetic coupling between different resonator elements.
In accordance with a first aspect of the present invention there is provided a filter element comprising a conductive element mounted in a conductive housing, the conductive element and conductive housing arranged such that the conductive element is electrically coupled to the conductive housing at one end of the element and capacitively coupled to the conductive housing at the opposite end of the element with a solid dielectric element disposed around a length of the conductive element.
This provides the advantage of smaller filters than equivalent conventional combline filters while still offering low insertion losses, high Q and good out of band performance.
Suitably the solid dielectric element is a ceramic element.
Preferably the solid dielectric element is in direct contact with the conductive element.
Most preferably the conductive element is plated onto the solid dielectric element.
Having the conductive element in direct contact with the solid dielectric element allows heat generated in the solid dielectric element to be dissipated through the conductive element. This provides good heat dissipation capability.
Preferably the solid dielectric element extends for substantially the whole length of the conductive element.
Preferably the capacitive coupling between the end of the conductive element and the conductive housing is adjustable.
In accordance with a second aspect of the present invention there is provided a filter element comprising an inner conductor having an electrical length less than a quarter wavelength of the resonant frequency of the filter and an outer conductor arranged as a transmission line; a solid dielectric element disposed between the inner conductor and outer conductor; wherein one end of the inner conductor is electrically coupled to the outer conductor, the opposite end of the inner conductor being capacitively coupled to the outer conductor.